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Full text of "On the distal end of a mammalian humerus from Tonbridge (Hemiomus major)"

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[From the Quarterly Journal of the Geological Society /or 
August 1899, Vol. Iv. | 

On the Distal End of a Mammalian Hdmerus from Tonbridge 
[HEiiioiiVS major). By Prof. H. G. Seeley, F.R.S., F.L.S., 
F.G.S. 

Mr. R. D’A. Anderson, of the Royal Indian Engineering College, 
has submitted to me the distal end of the right humerus of a 
mammal for determination. The bone was found in August 1898 
by Mr. GrenvLllo Anderson, on the bank of the River Medway near 
Tonbridge (at a time when the river was running very low), when it 
was seen projecting from the reconstructed rock. The locality is 
not far from Messrs. Curtis & Harvey’s gunpowder-mills, at a point 
between a broken and disused lock-basin and an old bridge near the 
ballast-pit. On visiting the spot I found fragments of flints among 
the materials which form the river-banks ; but although this might 
support a reference of the specimen to any geological period of 
subsequent date, there are conditions of mineral structure and 
osteological character which incline me to believe that the bone has 
been derived from the Weald Clay. 

When the fossil came under my notice, the distal end was broken 
from the shaft ; and the shaft was split, showing the very thin 
condition of the bone of the shaft, and the hard, sandy, caleareou. 
matter which filled the medullary cavity. Traces of matrix at the 
distal end show that the specimen has been derived from (]uartz- 
sand bound together with limonite, such as might occur in the 
Hastings Sand, Weald Clay, or Lower Greensand, but the character 
of this matrix is opposed to the possibility of the specimen being of 
post-Tertiary age. 

The fossil, as preserved, is 4 inches long, and indicates a humerus 
which may have been 6 inches long when perfect, as large as that 
of a wolf, but smaller than in a bloodhound. 

The shaft of the bone is flattened on the inner side, convex on the 
outer side, and thus it has a side-to-side compression approximating 
to half a cylinder, but is somewhat flaltened towards the olecranon- 
pit behind. It is rather obliquely flattened above the condyles 
in front, making the shaft | inch deep on the inner side at the 
distal end, and rather less on the outer side. The side-to-side 
measurement is least, as usual, above the distal articulation. Towards 
the proximal fracture the depth of the shaft, which is augmenting, 
is inch from front to back, while the side-to-side measurement 
is inch. 

The form of the shaft, flattened on the inner side, precludes any 
comparison of the animal with Carnivora, and indicates a resem- 
blance to Ungulate types. 

The distal articular condyles are set on to the shaft at a forward 
angle, which shows the animal to be terrestrial. When the shaft is 
held vertically, the condyles are anterior to it. There is no animal 
known to me in which this character is developed to the s uuc extent. 

The extreme width of the condylar end of the bono is 1 j inch. 
In narrowness of the condyles the character is somewhat ])ig-likc. 
The external surface of the condylar end of the bono is convex and 




Anterior aspect. Posterior aspect. Lateral aspect. 

noticed this character in any other animal. The condyles are 
rounded from above downward in front, but almost inappreciable 
posteriorly. They are divided in front by a moderately deep 
vertical groove, which suggests the camel ; though in that genus 
the external margins of the condyles are not rounded as in this 
fossil. The inner condyle is compressed from side to side to a blunt 
ridge, and this condition contributes to give the condyle a narrow 
aspect, which is exceptional. The outer condyle is flattened or 
traversed round its convex middle part by a very slight vertical 



414 PROP. H. G. SEELET ON THE DISTAL END [Aug. 1 899, 

narrow, without an appreciable lateral pit, a character of some 
interest, since all deer have the outer side of the condyle concave or 
with a central pit, while the vicuna (Auchenia) has a convexity. 

When the bone is held vertically and seen from the front, the 
condyles are oblique ; because the outer condyle, which is the 
larger, then descends lower than the inner condyle. I have not 

Hemiomus majos, gen. et sp. nov. : distal end of humerus, nat. size. 



B. S«eU7 dal. 



Yol_ 5^.] OF A MAMMALIAN HUMERUS FROM TONBRIDGE. 415 

depression. Above the condyles in front there is the usual supra- 
condylar depression, somewhat deep and narrow. 

On the posterior aspect the distal end shows its moat distinctive 
features. A long narrow olecranon-pit extends above the small 
external condylar surface. It is ovate in outline, filled with limonite, 
is inch long and fully | inch wide in the middle. The inner 
bord'er is comparatively straight and vertical, and the outer border 
is convex. 

The small external condylar surface below the olecranon-pit is an 
inconspicuous convexity about ^ inch deep. No part of the inner 
condyle is visible on the posterior aspect of the bone. 

The bone which margins the upper half of the olecranon-pit on 
the inner side is inch wide, rounded from side to side, but slightly 
worn. The external margin of the pit, which may also be a little 
worn, is about half as wide. This character of the relatively external 
position of the olecranon-pit is seen in the humerus of the tapir, 
and to some extent among horses. 

The lower half of the process which usually borders the olecranon- 
pit is absent in this fossil ; and on the inner side, in its place, is an 
ovate cartilaginous surface, concave in length, inclined obliquely 
inward, making an angle of about 45° with the axis of the shaft. 
There is a similar, but much smaller, atrophy of the corresponding 
process on the external side, giving rise to a truncated surface, | 
inch deep, forming a narrow crescentic impressed area. This 
character defines a lateral concavity between the hinder end of the 
humerus and the anterior border of the olecranon-process of the 
ulna. This truncation of these parts of the humerus is especially 
common in antelopes and deer ; but, in comparison with known 
types, recent and fossil, it is much more developed in the Tonbridge 
fossil animal. There is no Tertiary mammal in which the character 
is so conspicuous as in existing types. If Hyracoiherium or Plio- 
lophus has the distal end of the bone placed as far forward on the 
shaft, it entirely wants the truncation of the bone on the lower 
borders of the olecranon-pit ; and there is no other fossil genus 
with which the specimen has a closer affinity. This consideration 
is perhaps evidence against the fossil being derived from some 
Tertiary stratum and accidentally left where it was found, and so 
far is favourable to the specimen being of Wealden age. 

On the whole, the weight of evidence from comparison with other 
types appears to incline towards reference of the fossil to the Artio- 
dactyla, though there are almost a.s many points in common with 
the Perissodactylate humerus. In neither is the hinder part of the 
distal articulation of the bone comparable to this fossil. I therefore 
infer that it indicates a new family type. The teeth described by 
Mr. Smith Woodward and by Mr. Lydekker are indicative of much 
smaller mammals from the Wealden Beds, and the interest of the 
specimen now described is chiefiy in its size. It may be known as 
Hemiomus major, gen. et sp. nov., in reference to the absence of 
ossification of the hinder aspect of the distal end of the bone, and 
as indicative of its size.